2004 Adobe Hills Swarm - Background Information

The “Adobe
Hills swarm” is named after a topographic
feature in the vicinity of this sequence.
Seismologists refer to this sequence as a swarm for several reasons.
Unlike a typical mainshock-aftershock sequence, there is no obvious
mainshock in this sequence. As of 9/20/2004, the largest event, a
M5.5 quake, occurred nearly 16 hours after the sequence began. By
then, more than 31 earthquakes had been located. The
USGS
Quaternary Faults and Fold Database of the United States shows few
mapped faults in the epicentral region, but those to the vicinity are
referred to as Huntoon Valley
fault system.
These faults are Late Quaternary (<15 Ka) in age and have a strike of
N48°E with sinistral and normal
sense of motion. This trend of the mapped faults is similar
to the trend of seismicity and to the left-lateral planes in the
moment tensor solutions
of the larger events.

This image is a screenshot from CISN Display, showing seismicity in the last
hour (red), last day (blue) and last week (yellow) as of 03:00 PM on September 20th

Historically the region to the east between this sequence and the north shore of
Mono Lake has been quite seismically active. Since 1974, the CISN
(California Integrated Seismic Network) has located more than 3,200
quakes within the box defined by latitude 37.85º – 38.15º,
longitude -119.25º – -118.5º. During the past 20
years 170 quakes had magnitudes that exceeded 3.0.

During the 1980’s this fault system was quite active. A swarm began
June 29, 1980 with a M4.6 quake. The sequence developed into a
long-lived swarm that gradually died out by the end of the year.
During that sequence 75 quakes were located with magnitudes above
3.0. The system picked up again in mid-1980’s abated somewhat
by 1995, but continued on at moderate levels until this sequence.
Seismologists do not know when the Adobe Hills sequence will end, but
if past behavior is any guide, it could persist for weeks to months.

While the swarm behavior of the Adobe Hills sequence and its
proximity to the Mono-Inyo craters and Long Valley volcanic
field allows a volcanic origin, there is little geophysical
instrumentation in the region to support or refute this
hypothesis. Geologists have mapped basaltic cinder cones
in the vicinity of this Adobe Hills sequence, but they
have not erupted in the past 3 million years. While
swarm activity is sometimes associated with volcanic
unrest, swarms also occur in non-volcanic areas.
Of particular note are the swarms
that occur every few years in the vicinity of
Dublin, CA
in the eastern San Francisco Bay region.
The USGS is temporarily deploying GPS equipment in the
region to detect any anomalous
deformation that could be attributed to volcanic origins.
(Meanwhile, the status
at Long Valley caldera remains “green”.)